1. Field of the Invention
The present invention relates to a surface acoustic wave device in which a first filter having a relatively low passband and a second filter having a relatively high passband are connected, and more specifically, to a surface acoustic wave device in which the first filter is provided on a first piezoelectric substrate and the second filter is provided on a second piezoelectric substrate, which is different from the first piezoelectric substrate.
2. Description of the Related Art
Recently, the size of a branching filter used in cell phones or other electronic equipment is decreasing. In order to decrease the size of such a branching filter, a surface acoustic wave filter including a plurality of surface acoustic wave resonators has been used.
In a branching filter, for example, a transmission-side filter having a relatively low passband and, for example, a reception-side filter having a relatively high passband are connected. In this case, when the passbands of both filters are close to each other, the steepness of the filter characteristic must be increased at the high-frequency side of the passband of the filter having the low passband. In addition, the steepness of the filter characteristic must be increased at the low-frequency side of the passband of the other filter having the high passband.
Japanese Unexamined Patent Application Publication No. 4-369111 (Patent Document 1) discloses an example of a surface acoustic wave branching filter used for such an application.
FIG. 12 is a schematic plan view showing the surface acoustic wave branching filter described in Patent Document 1.
In a surface acoustic wave branching filter 101 shown in FIG. 12, in order to reduce the size and the cost thereof, a first filter 103 and a second filter 104 are provided on the same piezoelectric substrate 102. That is, the first filter 103 having a relatively low passband is provided by forming electrodes defining a plurality of surface acoustic wave resonators on the piezoelectric substrate 102. Similarly, the second filter 104 having a relatively high passband is provided by forming electrodes defining a plurality of surface acoustic wave resonators on the piezoelectric substrate 102.
Patent Document 1 also discloses a structure in which the first filter and the second filter that have different passbands are provided on different piezoelectric substrates.
However, Patent Document 1 does not describe the type of piezoelectric substrates that are used when the first filter in which the frequency range of the passband is relatively low and the second filter in which the frequency range of the passband is relatively high are provided on separate piezoelectric substrates.
As described in Patent Document 1, a structure in which a transmission-side filter and a reception-side filter are provided on the same piezoelectric substrate and a structure in which the transmission-side filter and the reception-side filter are provided on different piezoelectric substrates are known. In such a surface acoustic wave branching filter, a piezoelectric substrate having a cut angle at which the propagation attenuation factor has a minimum value is primarily used. When the transmission-side filter and the reception-side filter are provided on separate piezoelectric substrates, two piezoelectric substrates made of the same material having a cut angle at which the propagation attenuation factor has a minimum value are typically used.
However, in the structure in which the transmission-side filter and the reception-side filter are provided on the two piezoelectric substrates made of the same material, when the structure is used in, for example, a branching filter in which the passband of the transmission-side filter and the passband of the reception-side filter are close to each other, the steepness of the filter characteristic is not sufficient and it is difficult to achieve a satisfactory frequency characteristic.